Phototube



Dec. 18, 1934.

PHOTOTUBE FiledApri1 22, 1933 E. D. wlLsoN v 1,985,006v

Patented Dec. 18, 193fl UNITED STAT-Es PATENT oEFlcE VPno'ro'rUE Pennsylvania Application April 22, 1933, Serial No. 667,494

super-sensitive to the red region of the visible spectrum and have not been sensitive to the other sections of the visible spectrum.

In my copending case on photo-sensitive apparatus, Serial No. 524,878, filed March 24, 1931, I have described apparatus for color matching. It required .a photo-tube for this apparatus that would be sensitive to colors in the same degree as the average human eye, in order that the colors might be matched as they appeared to the average human eye.

This invention hasn particular application in the apparatus described in the copending case, but it is not limited to such an application. It

is apparent that such a photo-tube will have` various applications in view of the fact that the tube responds in the same degree to colors as does the average human eye.

The novel features that I consider characteristie of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and. its method ofV operation, together with additional objects and advantages thereof, will best be understood from the following description of a specinc embodiment when read in connection with the accompanying drawing, in which:

Figure 1 is a front elevation partly in section of the preferred structure of a photo-tube constructed in accordance with my invention;

Fig. 2 is a view in side elevation partly in section of the photo-tube of Fig. 1; and',

Fig. 3 is a graph showing the color sensitivity of my `photo-tube in comparison with that of the average human eye, and of the standard red sensitive photo-tubes of the prior art.

'I'he preferred structure of the photo-tube as illustrated in Figs. 1 and 2 consists of a suitable envelope of transparent glass 10 mounted upon ,the usual base 11 with the usual conductors 12, 13 and 14 extending through the press of the tube to the interior thereof. The press has preferably :dat sides 15 and centrally supports the (Cl.- Z50-27.5)

connection to the anode which consists of one or more wires or rods 16, 17, preferably extending through the central portion of the tube. At

its upper end the anode structure preferably sup ports a pellet or capsule 18. 'I'he cathode 19 may likewise be supported from the same press, and while this cathode may have various forms, it is preferable to utilize the semi-cylindrical form disclosed in the drawing, partially surrounding the major portion of the anode.

The pellet or capsule 18 generally has about 50 milligrams of material composed of one part caesium-dichromate to two parts of pure silicon. It should be capable of releasing 3 milligrams of caesium per square inch of cathode surface. 'I'he cathode may be of silver, or may have a. pure silver. surface. It is preferably constructed of solid silver 3 mils thick and has an area o f 2 square inches. Approximately a square inch of lead oxide or stannous oxide in a binder such as amyl acetate is painted upon the flat sides 15 of the press.

The tube is baked at a temperature of about 400" C. until the gases and vapors are driven out of the glass and a good vacuum is obtained therein. The tube is then cooled to room temperature and about four millimeters of pure oxygen is admitted. About volts direct current is then applied across the electrodes and produces a glow-discharge that oxidizes the silver surface of the cathode until the oxide which is first formed is broken down by the heat of the discharge. Complete deoxidation takes place in about 5 seconds. The oxygen is-then pumped out and a fresh supply of oxygen is admitted to the same pressure and the cathode is oxidized carefully until its color passes through a bright green into a reddish tinge. The tube is again exhausted. An induction coil is placed varound the tube and the pellet is flashed by induction currents to produce caesium vapor. The tube is then baked at a temperature between 225 and 275 C. until the reaction is completed between the silver oxide surface of the cathode and the caesium; This reaction is visibly indicated by the cathode turning to a uniform dull black color. The lead oxide or stannous oxide painted on the press is for the purpose of absorbing the excess caesium vapor. If the cathode has a bronze or coppery appearance, it indicates that there had been too much caesium released in the tube.V

` The baking is continued until the caesiumo'xide is almost completely broken down into higher oxides. This is visually indicated by the cathode becoming quite light to an ivory or cream appearance. If the cathode surface breaks up into orange colored spots, this phenomena indicates the presence of too much initial caesium for the type of tube.

The tube is cooled and preferably argon is admitted `at about 200 microns pressure. A glow-discharge, preferably at a current density not exceeding 30 milli-amperes is through the tube again from the anode to the cathode until by test the sensitivity rises to a maximumvalue. When this maximum value is reached, the discharge is then reversed so that it passes from the cathode to anode until the sensitivity rises to a new maximum. After this new maximum is reached the discharge is re-f versed again for a short period so that is passes from anode to cathode until the sensitivity reaches a stationary value. Thevtube -is then sealed off, unless a vacuum photo-tube is desired, in which case the argon is pumped out and' the tube sealed after a sufficient vacuum is obtained. y

In Fig. 3 is disclosed curves showing the relative sensitivity in regard to wavelength of the eye A, a photo tube constructed according to my invention described herein B, and a photo-tube sensitive to the red region C. The vertical scale of the eye is not drawn to the same scale as that of the tubes for the sake of clearness. It will be noted that my tube has a maximum sensitivity in the blue and central region of the spectrum vin place of in the red region.

Although I have shown and described a certain specic embodiment of my invention, I am aware that many modications'thereof are possible. My invention, therefore, is not tobe restricted except insofar as is necessitated .by the prior art and by the spirit of the appended claims.

passed I claim as my invention:

l1. I'he method of forming an electrode for a photo-electric tube which comprises oxidizing an electrode having a silver surface, heating said electrode surface in caesium vapor until itloses its black color, cooling-said electrode to about room temperature, andsubjecting it to glow discharge. n

2. The method of forming an electrode for a photo-electric tube which comprises oxidizing an electrode having a silver surface, heating said electrode surface in caesium vapor until it loses its blackcolor, cooling said electrode to about room temperature, and subjecting -it to glow discharge in an argon atmosphere until its light l sensitivity rises to a. maximum, then reversing the polarity of said discharge until the light sensitivity rises to a second maximum, and then again reversing the polarity of said discharge until said sensitivity becomes constant.

3. The method of making a silver-surfaced cathode of a photo-tube photo-sensitive which comprises oxidizing the surface of said cathode, releasing about said cathode substantially Blmilligrams ofV caesium per square inch of said cathcde surface..and then baking said cathode until the reaction is complete between the silver oxide surface of the cathode and the caesium.

4. The vmethod of making aV silver-surfaced cathode of a photo-tube photo-sensitive which comprises oxidizing the surface of said cathode,

' EARL D. WILSON. 

